Economic feasibility of methods for secondary and tertiary recovery of petroleum often depends on accurate measurement of the quantity and location of oil remaining in a formation after previous recovery processes have been completed. Such measurements are desirably carried out in "old" wells, i.e., in wells used to produce the formation. Reasons: (i) accuracy is increased; and (ii) costs are decreased; the process of drilling a new well displaces some fraction of the oil originally in the formation away from the hole, and it is desirable to evaluate the potential recovery from a reservoir without incurring the expenses of drilling a new well.
In my U.S. Pat. No. No. 3,817,328 for "Neutron Absorption and Oxygen Log for Measuring Oil Content of Formations", June 18, 1974, assigned to the assignee of this application, I describe a method for accurately determining the oil content of a reservoir containing both mobile oil and a significant gas saturation. The first step was the recording of the response of both a thermal-neutron-decay-time log and a neutron-activated-oxygen log to a formation traversed by a well bore. A purposeful change was then made in the oil saturation in a given region of the formation surrounding the well bore by injecting fluid under sufficient pressure to displace the connate fluids. This change could constitute the removal of substantially all the oil or the removal of as much oil as could be displaced by a proposed flooding technique. The combination of the thermal-neutron-decay-time log and the oxygen log was then run again to record the response of the same given region. The difference in the oil content around the well bore was determined from the differences between the two sets of logs.
My method may be somewhat limited, however, in some applications by the requirement that the oxygen activation log be calibrated at least to the extent that changes in log readings be proportional to changes in the oxygen content of the reservoir fluids with a predetermined single constant of proportionality. The response from logging tools currently available are unduly influenced--in some applications--by the pipe, cement, and liquids in the well bore; experience has shown that calibration valid at all depths in the well, is difficult (if not impossible) to achieve in such situations.
I am also aware of the contributions of others in the oil-content-measuring field. For example, pulsed-neutron-capture logs of the over-all cross-section for the capture of thermal neutrons by a rock matrix and reservoir fluids surrounding the well bore, have been proposed (see U.S. Pat. No. 3,562,523, J. E. Richardson and R. E. Wyman, or Pat. No. 3,631,245, J. R. Jorden, Jr. and F. R. Mitchell).
The above-mentioned methods for oil content measurement are only adequate, however, when the pulsed-neutron-capture cross-section of formation brine is known and is sufficiently different from that of oil. The methods have in common that they distinguish oil from formation brine through the greater absorption of thermal neutrons in the brine due to the presence of the chlorine nucleus therein. When the formation water is fresh, the pulsed-neutron-capture cross-section of the water and oil are so similar that the method loses sensitivity.
Furthermore, in many oil reservoirs, say the SACROC field in the West Texas region of the United States, while the produced brine may be saline enough that prior art methods could work, the brine in the formation often significantly varies between producing intervals. It has been common practice involving the injection of water into petroleum reservoirs for secondary recovery purposes (including the SACROC field) to inject brine with salinity different from that in the virgin reservoir. Thus, at the time that an accurate measurement of oil content is desired, the salinity of the water in the formation depends on the degree of flushing by injected water, which may be significantly different over the reservoir. If oil is known to be at a saturation so low that it is not changed by water flow, and there is no gas saturation, a logging measurement of saturation can be accomplished by injecting brine of known salinity prior to the start of the measurement.
The prior art has also provided several logs which serve as useful indicators of the presence of hydrocarbons, which can be run through casing, and which are based on measurements reflecting the abundance of elements other than chlorine. These include logs designed to reflect the activation of oxygen nuclei as well as logs designed to measure the inelastic scattering of nuclei by carbon and oxygen. Oxygen abundance can also be gained from background correction applied to some versions of pulsed-neutron-capture logs. But the recorded signals from each of the logging devices required to carry out the above-mentioned methods are only sensitive to the abundance of elements other than those of principal interest, and indicates the presence of oil rather than its abundance.
In summary, the existing prior art methods of which I am aware are not quantitative enough to provide accurate determination of the fractional content of oil in a reservoir rock containing oil, water, and gas, especially if the formation brine is non-saline and has been varied by previous waterfloods.